The above described PTC device is widely used as a protection device which protects an electric element (such as a secondary cell) constructing various electric apparatuses or a circuit constructing such electric apparatuses when an excessive amount of current passes through the apparatuses. Also, the PTC device is widely used as a protection device which interrupts a current flow through the apparatus when such electric element fails so that its temperature rises abnormally.
One example of such PTC device is shown in a schematic cross-sectional view in FIG. 1. A PTC device 10 comprises a PTC member 12 and leads 14 and 16 which are placed on the both sides of the PTC member. The illustrated PTC device 10 is located in, for example, a charge/discharge circuit of a secondary cell, and used as a circuit protection device to interrupt an excess current when such current passes through the circuit. In the illustrated embodiment, the lead 14 is electrically connected to a substrate which has a circuit protection circuit, while the lead 16 is electrically connected to a secondary cell side.
The PTC member 12 is composed of a PTC element 18 formed of an electrically conductive PTC composition and metal electrodes 20 and 22 which are placed on the both sides of the PTC element, and they are generally integrated by means of heat pressing.
The metal electrode 20 of the PTC member 12 is electrically connected to the substrate side lead 14 with solder paste which is sandwiched by the metal electrode 20 and the lead 14, and the metal electrode 22 is electrically connected to the cell side lead 16 via solder paste 24 which is sandwiched by the metal electrode 22 and the lead 16.
In order to produce such PTC device, the following is carried out for example: The solder paste 24 is placed on the substrate side lead 14, the PTC member 12 is place such that the metal electrode 20 is located on the solder paste 24, the solder paste 24 is place on the metal electrode 22, and the cell side lead 16 is placed on the solder paste so that an assembly of these members is formed. Thus formed assembly is passes through a reflow furnace so as to melt the solder paste followed by solidifying the molten solder paste by cooling, whereby the PTC device is obtained. In order that the assembly is passed through the reflow furnace, a jig having a plurality of concave portions is used. Each of the concave portions of the jig can receive and keep all the members superimposed in the above described order which form the assembly. The jig having a plurality of the assemblies formed by superimposing all the members in the concave portions is passed through the reflow furnace.
Such assemblies are passed through the reflow furnace to connect the members together via the solder, so that the PTC devices are obtained. Then, an oxygen barrier layer 26 is formed on the side of the PTC member. For such purpose, the obtained device is kept in other jig and an epoxy resin is applied onto the side of the PTC device to from a coating layer, followed by curing the layer to form the oxygen barrier coating 26.